In a number of industries, workers still visually inspect hot, glowing objects with their unprotected eyes. Direct exposure to infrared (IR) radiation, however, could cause physical injury to the workers. Accordingly, in some instances, light shields are worn which attenuate the radiation, thus providing some protection against IR exposure. However, the use of light shields often restricts the workers' mobility. For example, wearing a light shield may restrict their ability to physically interact with other objects that are not glowing, such as tools, controls and the like.
Conventional optical inspection devices have also been used to make observations/inspections of hot objects. For example, the so-called “passive method” utilizes a signal collector, either with CRT tubes, charge-coupled device (CCD) cameras, or IR cameras, to receive the self-emitted radiation from the hot objects. This approach is similar to the use of human vision, with the signal collectors essentially functioning as “eyes”. The passive method, however, is subject to a phenomenon known as the “Cavity Radiator Effect.” The Cavity Radiator Effect, postulated by Plank in 1900 and proved by Einstein in the early 20th century, can deceive visual observers as to the true nature of the object observed. More specifically, based on this principle, concave surface features of a self-radiating object appear to be nearly perfect black bodies; accordingly, they may be mistaken as convex features. Additionally, the “illumination” is self-emitted and thus often carries unwanted information. Images collected via this passive method are generally not suitable for automatic machine vision applications.
Another prior art approach, the so-called “active method” utilizes external lights that are projected onto the hot object. A camera is used to collect the reflected, as well as the self-emitted radiation from the hot surfaces. In the active method the idea is to overpower the self-emitted radiation with very strong external radiation. In other words, the reflected light is within the spectrum of the predominant self-emitted radiation, but is distinguishable based on its intensity. The external lights can be designed to highlight the surface information of interest such as contour and surface dimples. The external radiation can be provided by various light-generating devices such as high power lamps or lasers.
Several problems, however, are associated with the active approach. First, few light sources exist that can overpower the radiation emitted by an object at 1350° C. Second, the self-emitted radiation still represents a problem: it degrades the signal quality of the reflected radiation. The signal-to-noise ratio (external light/self-emitted light) is typically low unless a very powerful light source is used. Third, these external light sources may be undesirable in the work environment because they are so intense.
Lasers have also been used as a light source to overpower self-emitted radiation from hot objects. Lasers can deliver extremely high power densities to reduce the significance of the self-emitted radiation. For example, a copper-based laser (radiating at 550 nm) has been used to overpower the self-emitted radiation of laser welding pool (temperature at about 3000° C.), which typically radiates from 230 nm to long IR.
Another prior art approach uses YAG lasers (1060 nm) in arc welding (temperature at about 2500° C.), which typically radiates a spectrum of from 275 nm to long IR. However, the use of lasers poses substantial problems. While lasers deliver high power density, the areas illuminated by the laser beams are small. Consequently, raster scanning is typically required when lasers are used as illumination sources. Moreover, these high power lasers are expensive, bulky, and pose various risks. And, in order to operate a laser-based system, the users must be protected with light shields and other protective equipment.
The use of infrared (IR) sensors or cameras in a passive method vision system are also of limited value due to several factors. First, IR sensors/cameras provide significantly less pixel resolution than their CCD counterparts. Second, IR radiation cannot be focused as well as visible light due to its wavelength. Third, using IR sensors/cameras does not solve the problems associated with illumination or the Cavity Radiator Effect previously described.
There have been attempts to use a combination of passive and active methods, but this approach does not resolve the issues posed by the Cavity Radiator Effect and self-emitted radiation.
In the past, the difference between IR and visible light has been the focal point of solving the problems associated with the glare of hot objects. This approach is ill-conceived because a hot object can radiate with both IR and visible light radiation. For instance, steel radiates at 650 nm at 1200° C.; that is, steel can radiate in RED as well as IR. In addition, if the self-emitted radiation is not removed from the collected signal, the noise caused by the self-emitted radiation impairs the system's ability to gather detailed and accurate information about the hot object. The prior art lacks an effective means of removing the self-emitted radiation from the collected signal of a hot object. Finally, it is also believed that none of the devices enabled by the prior art is portable. This fact has limited the utility of such devices for certain applications. A portable device would be desirable for users who need to look at hot objects, but who do not need to take quantitative measurements. The external light sources used in prior art devices are too powerful and/or heavy to be low-risk and portable. In summary, the prior art approaches have been of limited value. The present invention overcomes these problems.